Basicity of amines.pptx

Organic chemistry

1. Basicity of amines Amines are basic because they possess a pair of unshared electrons, which they can share with other atoms . These unshared electrons create an electron density around the nitrogen atom. The greater the electron density , the more basic the molecule. Groups that donate or supply electrons will increase the basicity of amines while groups that decrease the electron density around the nitrogen decrease the basicity of the molecule. Due to this property of the alkyl group attached to the nitrogen of the amine, the electron density around the nitrogen atom increases due to which its electron releasing ability increases. Due to the greater and easier release of electrons in the alkyl amine, the molecule becomes more basic than ammonia . Whereas in ammonia, no such releasing group attached. So, obviously the basicity of ammonia is much less than that of an alkyl amine.

2. The differences in the basicity order in the gas phase and aqueous solutions are the result of solvation effects. Amines in water solution exist as ammonium ions. - In water, the ammonium salts of primary and secondary amines undergo solvation effects (due to hydrogen bonding) to a much greater degree than ammonium salts of tertiary amines. These solvation effects increase the electron density on the amine nitrogen to a greater degree than the inductive effect of alkyl group.

3. Comparison in the basicity of the aliphatic amines, the basicity of tertiary amines to be the greatest, followed by secondary amine and then primary amine. But this is not so. The order of basicity is NH3 < primary amine ~ tertiary amine < secondary amine…..Explain Why? This is due to (i) Steric hindrance effect and (ii) Solvation of ions effect This “unexpected” order of basicity is due to Steric hindrance: - The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. - So, the more the number of alkyl groups attached, lesser will be its basicity. So, the decreasing order of basicity will be: Primary amine > secondary amine > tertiary amine.

4. Solvation of ions When amines are dissolved in water, they form protonated amines. Also , the number of possibilities of hydrogen bonding also increases. More the number of hydrogen bonding, more is the hydration that is released in the process of the formation of hydrogen bonds . - The more the hydration energy of the molecule, more is the stability of the amine. Experiments have indicated, in terms of hydration energy, that the tertiary amines are the least stable followed by the secondary amines followed by the primary amines. - This can be explained on the basis of the following reactions. In primary amine: RNH 2 + H 2 O →→ RNH 3 + + OH In secondary amine: R 2 NH + H 2 O →→ R 2 NH 2 + + OH In tertiary amine: R 3 N + H 2 O →→ R 2 NH+ + OH- - So , the number of hydrogen bonds possible when primary amines are dissolved in water is the greatest, implying that they are most stable species of amine, the least being the tertiary amines. - The combined effect of the pushing effect of the alkyl group (+ I effect), steric hindrance and the salvation of amines causes the basicity order to be: (basicity of tertiary is almost the same as that of primary).

5. WHY ARYLAMINES ARE WEAKER BASES THAN CYCLOHEXYLAMINES? In Aniline ( Arylamines ), the lone pair on nitrogen (which is free for protonation in a cyclohexylamines ) is present in conjugation with the benzene ring, so aniline can't lose electrons as easily as a cyclohexylamines can. The basicity of amines is due to the nitrogen of the amine donating its lone pair of electrons.

6. Solubility of Amines in water The small amines of all types are very soluble in water. - All of the amines can form hydrogen bonds with water - even the tertiary ones. Although the tertiary amines don't have a hydrogen atom attached to the nitrogen and so can't form hydrogen bonds with themselves, they can form hydrogen bonds with water molecules just using the lone pair on the nitrogen. Solubility falls off as the hydrocarbon chains get longer - noticeably so after about 6 carbons. The hydrocarbon chains have to force their way between water molecules, breaking hydrogen bonds between water molecules. However, they don't replace them by anything as strong, and so the process of forming a solution becomes less and less energetically feasible as chain length grows. The water solubility of 1º and 2º-amines is similar to that of comparable alcohols. As expected, the water solubility of 3º-amines and ethers is also similar. These comparisons, however, are valid only for pure compounds in neutral water.

7. BETWEEN ALCOHOLS & AMINES, WHICH ONE IS MORE SOLUBLE & WHY? Alcohols are more soluble in water due to the electron availability of the two electron pairs in the oxygen which can accept a proton. - And simultaneously, The hydrogen bonds between the Oxygen (lone pair of electrons) in the alcohol and the water molecules is stronger because Oxygen (lone pair) is more electronegative than the Nitrogen (lone pair) in the NH 2 group in the amines

8. WHAT IS THE RELATIONSHIP BETWEEN HYBRIDIZATION AND ACIDITY OR BASICITY? For acidity, the hydrogen should be attached to an electronegative atom. The more electronegative the atom is, the more acidic that hydrogen is . - Electronegativity depends on %s character. More the %s character, more the electronegativity of central atom. So, sp³ orbital has the least electronegativity and “ sp ” orbital is most electronegative. Hence , it follows that alkynes ( sp hybridised carbon) are more acidic than alkenes The reason for this is that since the s-orbital is spherical, electrons in it experience more force of attraction from the nucleus. Since electronegativity is largely associated by how tightly valence electrons are held, as s character of a hybrid orbital increases, its electronegativity increases and hence its acidity increases. Order of basicity is reverse order of acidity. This result is realised when the Electronegativity of sp -carbon is greater than that of sp³ nitrogen .

9. WHY ARE AROMATIC AMINES LESS BASIC THAN ALIPHATIC AMINES? The basicity of amines is due to the nitrogen of the amine donating its lonepair of electrons. In aromatic amines this lone pair is in conjugation (delocalized) with the aromatic ring. While in aliphatic mine this doesn’t happen and thus the lone pair is ready to be donated. Aromatic amines are weaker bases than aliphatic amines. This is because the amine donates its electron density to the aromatic ring. Also, the amine forms stable resonance structures with the aromatic ring, which is absent once the amine becomes protonated . Electron donating groups on the aromatic amine increase the basicity of aromatic amines. This is because the electron donating groups contribute to the electron density on the nitrogen . Electron withdrawing groups on the aromatic amine decrease the basicity of aromatic amines. This is because the electron withdrawing groups steal electron density from the nitrogen . Anything ortho to the amine, no matter whether it is electron donating or withdrawing, will decrease the basicity of the aromatic amine. This is because of the ortho effect, which is basically sterics . The protonated amine will have a greater steric interaction with the ortho group, so it will be less stable.

10. Boiling Point of Amines The boiling points and water solubility of amines is depend on hydrogen bonding. The powerful intermolecular attraction that results from -O-H---O- hydrogen bonding in alcohols whereas -N-H---N- hydrogen bonding is weaker, as the lower boiling points of similarly sized amines. Whereas 1, 2 & 3-amines , has different boiling point, Since 1 amines have two hydrogens available for hydrogen bonding, to have higher boiling points than isomeric 2 amines , which in turn should boil higher than isomeric 3 amines (no hydrogen bonding). Indeed , 3 amines have boiling points similar to equivalent sized ethers; and in all but the smallest compounds, corresponding ethers, 3 amines and alkanes have similar boiling points.

11. The reason for the higher boiling points of the primary amines is that they can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions. Hydrogen bonds can form between the lone pair on the very electronegative nitrogen atom and the slightly positive hydrogen atom in another molecule. The boiling point of the secondary amine is a little lower than the corresponding primary amine with the same number of carbon atoms. Secondary amines still form hydrogen bonds, but having the nitrogen atom in the middle of the chain rather than at the end makes the permanent dipole on the molecule slightly less. The lower boiling point is due to the lower dipole-dipole attractions in the dimethyl amine compared with ethylamine. The trimethyl amine has a much lower boiling point ( 3.5 C ) than 1-aminopropane ( 48.6 C ). In a tertiary amine there aren't any hydrogen atoms attached directly to the nitrogen. That means that hydrogen bonding between tertiary amine molecules is impossible. That's why the boiling point is much lower.

12. COMPARING THE BASICITY OF ALKYL AMINES TO AMIDES. The basicity of alkyl amines to amides, with an alkyl amine the lone pair electron is localized on the nitrogen. However, the lone pair electrons on an amide are delocalized between the nitrogen and the oxygen through resonance. This makes amides much less basic compared to alkyl amines. General principles of basicity Amines are basic. They like to gain a proton. R-NH2 → R-NH3 +. It is very difficult for neutral amines to lose a proton. An amide, however, can lose a proton much more easily. This is because the carbonyl group next to the nitrogen contributes to a resonance structure that places the negative charge on the oxygen. Thus, the negative charge of the conjugate base is distributed over both nitrogen and oxygen.

13. EXPLAIN THE STEREOCHEMISTRY OF AMINE 3 amines can be chiral. But they are always racemic because of spontaneous inversions at room temperature. Even protonated 3 amines undergo inversion because the proton comes on and off in an acid-base equilibrium. 4 amines can be chiral and they stay chiral because they don't undergo inversion.

14. Hinsberg Test Hinsberg reagent is an alternative name for benzene sulfonyl chloride . Hinsberg test for the detection and distinction of primary, secondary, and tertiary amines in a given sample. The reaction of the benzene sulfonyl chloride with secondary amines gives a sulfonamide product that is NOT soluble in alkali. The reaction of the benzene sulfonyl chloride with primary amines gives a sulfonamide product that is soluble in alkali . Tertiary amines – Do not react with the benzenesulfonyl chloride in the Hinsberg test solution and thus initially provide an insoluble solid or oil (the unreacted amine) which, on acidification with HCl, dissolves to give a clear solution of the amine salt.

15. Nitrous acid test Nitrous acid ( HNO2 or   HONO) reacts with aliphatic amines in a fashion that provides a useful test for distinguishing primary, secondary and tertiary amines . 1°-Amines + HONO (cold acidic solution) →→ Nitrogen Gas Evolution from a Clear Solution 2°-Amines + HONO (cold acidic solution) →→ An Insoluble Oil (N-Nitrosamine) 3°-Amines + HONO (cold acidic solution) →→ A Clear Solution (Ammonium Salt Formation)

16. DISTINGUISH BETWEEN ALIPHATIC PRIMARY AMINES & AROMATIC PRIMARY AMINES. At low temperature (0 – 5 C ) aromatic primary amines dissolved in strong acids (HCl & H 2 SO 4 ) reacts with nitrous acid (NaNO 2 +HCl) to form water soluble diazonium salts. Aliphatic primary amines do not form stable diazonium salts under similar condition. They react with nitrous acid to yield alcohols and nitrogen (causes rapid foaming).